I. Field of the Invention
This invention relates to the coating of strip material, particularly metal sheet, with protective and/or decorative layers of solids while avoiding the addition of liquids such as solvents, softeners, suspension media, or the like.
II. Description of the Prior Art
Metal sheet material, for example thin aluminum strip used for beverage cans and other purposes, is frequently coated with organic films to provide surface protection and/or decorative finishes. The coatings are typically applied by dissolving or suspending polymers and other components in organic solvents, applying the resulting mixtures by roller coater or doctor blade to the strip, and baking the resulting product to remove the solvents and to cross-link the polymer.
Unfortunately, the solvents emitted during this conventional procedure cause environmental problems, thus necessitating the use of expensive pollution control systems and complex ovens to avoid the build-up of flammable vapors to explosive concentrations. Also, in order to ensure that the coating polymers will properly dissolve, it is often necessary to use lower molecular weight polymers than would be desirable for providing ideal coating properties.
As an alternative to roller coating, so-called "falling film" extrusion coating of aluminum foil and paper with molten polymer coating materials is well known. The thickness of the coating is normally controlled by extruding the molten polymer from a slot in an extrusion head positioned several centimeters above a moving strip in the form of a film having a grater thickness than that finally required and then thinning the film by stretching it as a free (unsupported) film under the combined effect of gravity and tension before applying it to the surface of the strip. This demands special rheological characteristics of the coatings material so that it can stretch without breaking. It is also very difficult to achieve thicknesses as low as 2 to 7 microns that are typical for aluminum packaging applications, such as aluminum beverage can ends.
An alternate means of controlling coating thickness during extrusion coating is to employ an extrusion die movably connected to a supporting structure, having an extrusion opening and die lips of a suitable shape positioned around the extrusion opening. The die lips are moved close to the strip an the clearance between grip and die lips is precisely controlled by adjusting the position of the extrusion head relative to the supporting structure. In such an arrangement, the thickness uniformity of the coating depends on the precision used in the manufacture and control of the die and the precision of the support roll normally used to support the sheet material during coating, as well as the uniformity of the metal gauge along the strip, and it proves very difficult in practice to produce uniform coatings of the desired thickness in an acceptable manner. For example, if a mechanical spacer, such as a roller, is used to maintain a uniform clearance between the die lips and the strip, unsightly marks may be made on the surface of the strip by the spacer and the marks may not be completely hidden by the applied coating.
An apparatus and method suitable for single-sided coating of a sheet material without reliance on mechanical spacers that contact the strip is disclosed in U.S. Pat. No. 4,675,230 of Jun. 23, 1987, assigned to the same assignee as the present application (the disclosure of which patent is incorporated herein by reference). Moreover, a related apparatus and method of two-sided coating of sheet material is disclosed in pending U.S. Pat. application Ser. No. 08/068,990, filed May 27, 1993 and assigned to the same assignee as the present application (the disclosure of which application is also incorporated herein by reference). The types of apparatus disclosed in this patent and patent application relay on the hydrodynamics of the coating material as it is applied to the strip for control of the film thickness and can readily compensate for variations in the gauge of the strip and any eccentricity of the support roll. This is achieved by using a coating head having a slot and an extended surface on the downstream side of the slot forming an angle with the moving strip converging in the direction of the strip travel. The extended surface directly contacts the coating material as it is applied to the strip, thereby generating hydrodynamic forces that cause the head to "float" on the layer of coating as it is being applied. Direct contact between the strip and the coating head is thus avoided, and this in turn avoids damage to or defacement of the metal or pre-coated metal surface to which the coating is applied.
The problem with devices of this particular kind is that, while they can generally handle coating materials having viscosities that are greater than the viscosities of coating materials applied by other coating techniques, for example conventional roller coaters, they still require the coating material to be of fairly low viscosity, so it has been necessary to dissolve or suspend the polymeric coating material in a suitable solvent, thereby creating the difficulties mentioned above.
There is therefore a need for a method and apparatus capable of coating a strip material with a polymeric coating layer in an efficient and convenient way without resort to the use of polymer solutions or dispersions.